plumbing and step issues.

Panic stations. First time checking all the plumbing. Emptied the kitchen sink, and it drained into the shower tray. Slowly.
The waste tank has a vent hose fitted. all plumbing done with 20mm convoluted pipe.
Have fitted a check valve today and made sure that all the pipes are level. Hope it fixes the issue. I will find out in the morning.

If you have 'Teed' the sink and shower drain then this is quite likely the problem. My simple answer would be take both to the grey waste tank separately. You could say 'been there, done that'.